Transfer cases used on vehicles equipped with four-wheel drive generally include; transfer gear shift means for shifting a transfer gear assembly between a high and a low speed drive range so as to transmit driving power with or without torque multiplication from an engine to wheels; center-differential gear shift means for shifting a center-differential gear assembly between a differential-locked and a differential-free drive mode; and, if used on vehicles equipped with part- time four-wheel drive, means for shifting the center-differential gear assembly between a four-wheel and a two-wheel drive mode. In any case, these shift means are needed to be operated within the driver's compartment to selectively shift the transfer case to a desired drive mode of operation.
One such transfer case is disclosed in, for example, Japanese Utility Model Publication No. 61-9791 entitled "2-4 Drive Mode Shifting Mechanism for a Four Wheel Drive Vehicle", published Mar. 28, 1986. The transfer case taught in the above mentioned Japanese Utility Model Publication has a hydraulic actuator for causing means which selectively shifts a center-differential gear assembly between a two-wheel and a four-wheel drive mode and a solenoid valve by which the hydraulic actuator is controlled in operation. The solenoid is electrically controlled to cause the hydraulic actuator so as to shift the center-differential into the two-wheel drive mode when the vehicle is traveling at a high speed. On the other hand, when the vehicle is traveling at a low speed, the center-differential is shifted into the four-wheel drive mode when a transfer is in a low gear or is selectively shifted to a desired drive mode between the two-wheel and four-wheel drive modes by manually operating a drive mode selecting switch operated by the driver.
Meanwhile, vehicles with electric shifting means for shifting transfer between a high and a low speed drive range are needed to be equipped with a synchromesh mechanism. Once a vehicle is in motion, the drive line will turn the output shaft continuously. As a result, a sliding gear will be whirling. When an attempt is made to mesh it with any of the cluster gears, the gear teeth will be subjected to damaging impact force. For one gear to mesh with another quietly and without damage, most gears must be rotating at nearly the same speed. For this reason, most transfers are equipped with a so called synchromesh mechanism.
Most synchromesh mechanisms practically equipped on the four-wheel drive vehicles are, however, hard to synchronize one gear to another with certainty in order to shift the transfer between a low and a high speed drive range while the vehicle is traveling. For an infallible shifting of transfer, there is proposed a transfer equipped with no synchromesh mechanism. Such a transfer is shifted between high and low speed drive modes by shift means actuated by an electric actuator. For effecting the shifting of the transfer, the vehicle is stopped and an engine clutch is disengaged to disconnect driving power from the engine to wheels.
It is a confusing operation to use such a switch for causing the actuator to change driving speed ranges which is to be operated by stepping a clutch pedal. The driver has a feeling of incompatibility with such a pedal and switch operation for changing drive modes. Furthermore, a problem in association with the transfer case without synchromesh mechanism is that a large force is needed to mesh one gear to another due to no rotation of gears.
The above mentioned confusing operation can be avoided by using mechanical shift means operated by shift levers one individual to each of a transfer and a differential. Such a mechanical shift means, because an operating force is generally different between transfer shifting and differential shifting operations, gives the driver a troublesome shift operation and an unfavorable operational feeling.
The above problems may be possibly avoided by use of motor driven shift means actuated through switches to shift both the transfer and differential. However, not only because of a large force generally needed for shifting the transfer which needs an expensive high power electric motor but because the provision of two motors is costly.